Electrical resistivity well logging systems and methods



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ELECTRICAL RESISTIVITY WELL LOGGING SYSTEMS AND METHODS Filed Oct. 18, 1948 4 Sheets-s116812 l vAouuu TUBE I MusulmansL I, 33

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Jan. 10, 1956 W D, MOUNCE 2,730,672

ELECTRICAL RESISTIVITY WELL LOGGING SYSTEMS AND METHODS Filed Oct. 18, 1948 4 Sheets-Sheet 4 "4 FIG. Il.

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nited States Patent ELECTRICAL RESISTIVITY WELL LOGGING SYSTEMS AND METHODS Whitman D. Mounce, Houston, Tex., assignor, by mesne assignments, to Esso Research and Engineering Company, Elizabeth, N. J., a corporation of Delaware Application October 18, 1948, Serial No. 55,046

16 Claims. (Cl. 324-1) This invention relates to electrical systems and methods for making resistivity logs in boreholes in the earth, such as oil wells and more particularly vto those which eliminate, to some degree, errors inherent in present arrangements for making resistivity logs.

In the making of resistivity logs in boreholes it has been common practice for some time to use what is known as a three-electrode system to make one of the curves or traces. It has also been common practice to associate with the log made with three electrodes a log made with a single electrode. In the patent to Mounce No. 2,376,- 168 granted May 15, 1945, for Well Logging an arrangement is shown and described for making both a three-electrode resistivity or impedance curve and a single electrode impedance or resistivity curve. The apparatus shown in said patent has provision for switching the electrode arrangement so as to make one log or the other.

While the curves mentioned in said patent are known to those skilled in the art as a three-electrode curve and a single electrode curve, actually they are used in connection with an additional electrode which is grounded at the surface of the ground, as by placing it in the slush pit. Thus in reality the systems employ four electrodes and two electrodes respectively. However, throughout this specification the terminology employed is that commonly used in the field. The terminology is also the same as that used in the articles by Hubert Guyod, entitled Electrical Well Logging appearing in the Oil Weekly between August 7, 1944, and December 4, 1944.

The present invention is here shown and described in this specification as applied to a three-electrode electrical logging system. A current electrode device is lowered into a borehole and the amount of potential difference which exists in the borehole due to current iiowing 'from this current electrode device is determined by two potential electrodes or rings located a small distance from the current electrode device. It will be obvious to those skilled in the art that while the specification discloses the invention as applied to a three-electrode system it may also be used with other electrode configurations.

A three-electrode system, that is, one having a single current electrode device and two potential electrodes is commonly called a standard arrangement of electrodes in the making of resistivity logs. See, for example, the Oil Weekly for October 23, 1944, pages 49 and 50. With it, as well as any other electrode arrangement heretofore devised, certain inherent defects or anomalies are present. These are particularly noticeable when the electrodes pass boundaries of earth formations. One of the purposes of making logs of wells is to locate these boundaries accurately and it is annoying to iind errors creeping into the curves just at these points.

A single electrode impedance system is better than a multiple electrode system for depicting boundaries, but it is defective in other respects. The magnitude of its deection does not give geologists the information they desire as to the magnitude of resistivity of a particular formation, Thus it has become the practice to make sevice eral resistivity curves on a well, each curve being used for a different purpose and requiring considerable skill and experience to interpret. These and other properties of electric logging systems are well explained in the articles by Guyod cited above.

It is an object of the present invention to devise an electrical system which will accurately record not only the boundaries between adjacent formations of the earth but give good indications of the apparent resistivity of the individual formations.

This and other objects may be attained by providing an electrical resistivity logging system in which the distance, either actual or effective, between electrodes in the borehole is changed as the electrical log is being made. The sequence of the changes in distance are such that certain inaccuracies or anomalies are removed from the log.

Other objects and advantages reside in certain novel features of the arrangement and construction of the systems as will be more apparent from the following description taken in connection with the accompanying drawings in which Fig. 1 is a somewhat diagrammatic view partially in cross section of an electrode assembly being lowered into a borehole, together with suitable apparatus at the surface of the ground for making an electrical resistivity log in accordance with the more basic principles of the present invention;

Fig. 2 is a circuit diagram of the apparatus of Fig. 1 and showing the connections to the electrodes in the borehole and the single ground at the surface;

Fig. 3 is a circuit diagram and associated apparatus of a three-electrode resistivity logging system similar to that shown in Fig. 1 but in which only a single conductor cable is employed;

Fig. 4 is a simulated diagram illustrating the ideal resistivity log and showing the kinds obtained with two systems of the prior art, with anomalies somewhat exaggerated, as well as systems constructed in accordance with the present invention;

Fig. 5 is a diagrammatic illustration of a still further embodiment of the invention in which the electrodes are automatically moved relative to one another as an electric log is being made so as to make a continuous resistivity curve of high fidelity;

Fig. 6 is a view in perspective of a control apparatus adapted for use with the logging arrangement of Fig. 5;

Fig. 7 is a view in perspective of a galvanometer adapted for use with arrangement of Fig. 5;

Fig. 8 is a circuit diagram of apparatus shown in Fig. S together with the electrical arrangement for recording an electric log using the apparatus of Figs. 6 and 7;

Fig. 9 is a diagram showing an arrangement of electrodes in a well and illustrating the connections to the electrodes to make a resistivity curve of high delity while using electrodes which are not physically moved relative to one another during the making of the logs but in which an electrical switching arrangement is employed to connect one or another current electrode in the system, thereby accomplishing what might be termed an ei'rective sequence of changes in the distance between electrodes;

Fig. lO is a diagrammatic representation of a switching arrangement associated with the reel which carries the cable of an electrical logging system, the switching arrangement being used to cause certain connections to the electrodes as shown in Fig. 9;

Fig. 11 is a view in perspective of a galvanometer designed to be used with an arrangement for making an electric log by effecting the connections shown in Fig. 9; and

Fig. 12 is a circuit diagram of an electrical system using a single conductor cable and in which circuit connections are effected as shown in Fig. 9 for making a continuous electrical resistivity log of high fidelity.

Referring to the drawing in detail and first to the ernbodiment of the invention shown in Figs. l and 2, it will be seen that suitable electrical apparatus is there illustrated as lowered into a borehole designated 15. The portion of the apparatus which is lowered into the borehole is shown below the indication 18. It may consist of a suitable housing 16 provided with a current electrode device C insulated from the housing 16 and a pick-up electrode device consisting of two potential or pickup electrodes designated P1 and P2.

An arrangement is provided for changing the distance between the current electrode device and the pickup electrode device. The pickup electrode device is physically supported on the housing 16 by means of two conductors 20 which are spooled upon a reel 21 located within the housing. The reel is provided with suitable means such as slip rings 22 and 23 for making electrical connections between conductors 20 and brushes 24 and 25 connected to conductors of the cable 26. The cable is used to lower the housing 16 and the electrodes P1 and P2 into the borehole.

For driving the reel 21 a Selsyn motor 27 is mounted in the housing 16, this motor being arranged to drive the reel through a suitable speed reduction gear box 28 and bevel gearing 29.

As shown in Fig. 1 the cable 26 is provided with six conductors. Two of these conductors designated 30 and 31 connect the pickup electrodes P1 and P2 to a vacuum tube measuring equipment designated generally 32, which may be conventional. The measuring equipment is provided with a galvanometer 33 which deects a light beam from a light bulb 34 upon photographic film 35 which is driven over a suitable driving spool 36 by a Selsyn motor 37, in synchronism with the Selsyn motor 27 in the borehole. A source of alternating current designated 38 has one terminal grounded as shown at 39 while the other terminal is connected to an electrical conductor 40 of the cable. The source 38 thus supplies current to both Selsyn motors 37 and 27. It also supplies current to the current electrode device C on the housing 16 in the borehole, this electrode device acting as a ground for the Selsyn motor circuits. The Selsyn motor circuit thus constitutes a control circuit for correlating the changes in distance between the current electrode device and the pickup electrode device with the readings of potential by the measuring equipment.

The connections for the arrangements shown in Fig. l are as illustrated in Fig. 2. The source of alternating current 38 is connected in series with a held coil within the Selsyn motor 37 as well as in series in the Selsyn motor 27 and the current electrode device C in the borehole.

It will be seen that with the system shown in Figs. l and 2, an arrangement is provided for moving the potential electrodes P1 and P2 with respect to the current electrode device C as an electrical log is being made. The purpose of such an arrangement will be described presently, but first another embodiment of the invention will be described.

Fig. 3 illustrates diagrammatically a form of apparatus in which only a single conductor cable is employed for accomplishing the same purpose as the six conductor cable of Figures 1 and 2. Parts of Fig. 3 which correspond to those of Figs. 1 and 2 are similarly designated. Thus the current electrode device in the borehole is designated C and the two potential or pickup electrodes are designated P1 and P2. The potential electrodes P1 and Pz are connected to slip rings 22 and 23 as in Fig. 1, and it will be understood that the conductors 20 which connect the slip rings 22 and 23 to the potential electrodes are wound upon a suitable reel such as shown at 21 in Fig. l. As illustrated in Fig. 3 reel 21 is actuated by a solenoid ratchet arrangement, the solenoid being designated 42 and the ratchet wheel being designated 43. This constitutes a control circuit and the design is such that each time the solenoid 42 is energized the reel 21 is rotated a small amount. Thus the potential electrodes P1 and P2 are advanced toward the current electrode device C a short uniform distance each time the solenoid 42 is energized. The brushes 24 and 25 which contact the slip rings 22 and 23 are connected to the primary of a transformer 44, the output of which is connected to a rectifier 45 which changes the alternating current signal to direct current and feeds it to the single conductor 46 of the cable through a connection 47. It will be apparent that the direct current fed to the conductor of the cable 46 will be proportional to the alternating current difference in potential across the pickup electrodes P1 and P2 caused by the passage of alternating current into the fluid of the borehole and into the surrounding earth formations from the current electrode device C. The conductor 47 may be provided with a choke or low pass filter 48 to keep alternating current out of the rectifier circuit.

Whereas the system of Figs. 1 and 2 had only a single source of alternating current the system of Fig. 3 is provided with two sources, one being for example 60 cycles and the other 50() cycles. The 500 cycle source is shown at 49 and the 60 cycle source at 50. Of course, other frequencies may be employed but these have been shown in Fig. 3 because they are commonly used for electrical logging in systems and their constants have been worked out. A 500 cycle source is commonly used for making a resistivity curve while a 60 cycle source is used to control the operation of switches or the like in a borehole. Thus in the system shown in Fig. 3 the circuit for the 500 cycle source 49 includes a conductor 51, a resistance element 52, a 500 cycle pass lter 53 and an electrical switch 54. Through these the source 49 is connected to the conductor 46 within the cable which passes into the borehole. In the electrode assembly in the borehole current from source 49 can pass through a second 500 cycle pass filter 55 to the current electrode device C.

The 60 cycle source is nonnally not connected to the conductor 46 of the cable. It is connected only when the switch 54 is actuated to engage its lower contact 56. The 60 cycle current can then pass through the conductor 46 and the 60 cycle pass filter 57 in the borehole to the solenoid 42 and ground. The direct current signals or variations in voltage coming up the conductor 46 of the cable from the rectifier 45 pass through the choke or low pass filter 58 and then to a D. C. vacuum tube voltmeter designated generally 59 and thence to ground at 60. These signals are traced upon a suitable chart 61 which is driven by a suitable solenoid ratchet arrangement, the solenoid of which is shown at 62 and the ratchet at 63. Through a suitable gear box 64 this ratchet mechanism 62-63 also drives the pointer 65 over the resistance elcment 52. Thus the effective portion of thc resistance elcment 52 is varied as the chart 61 moves. It will be observed that the resistance element 52 is shown as irregular. That is, there is greater increase in resistance per unit distance of travel of the pointer 65 as the pointer rotates in a clockwise direction. The resistance element 52 with its pointer 65 and its driving mechanism may be termed a correcting device, the purpose of which will be explained hereinafter in connection with the explanation of the errors in curve (d) of Fig. 4.

In the arrangement of Fig. 3, the circuit is such that the solenoid 42, which moves the potential electrodes P1 and P2 in the borehole, is energized simultaneously with the solenoid 62 which drives the chart for the recording voltmeter 59. The chart and the pickup electrodes move in synchronism. Any suitable means may be provided for this correlation. In the arrangement illustrated a relay coil 66 is provided which closes a switch 67 simultaneously with the actuation of the switch 54 which controls the passage of 60 cycle current to the conductor 46 of the cable. The switch 67 is normally open but when the relay 66 is energized it engages the contact 68 so that the circuit for a battery 69 is completed from a ground 70 to the solenoid 62 which drives the chart. The relay coil 66 is energized periodically by means of a rotating switch 71 provided with a contact button 72 which engages the blade 73. The rotating switch 71 may be rotated by hand or by any suitable means to cause pulses of current from a battery 74 to iiow through the relay coil 66. Whenever the circuit to the relay coil 66 is closed both switches 67 and 54 are pulled toward the relay coil and engage their lower contacts 68 and 56, respectively. Thus each time that the solenoid coil 62 is energized the solenoid coil 42 in the borehole is also energized.

In accordance with the invention it is possible to make continuous logs, but for purposes of explanation of the fundamental principles the arrangements of Figs. l and 2 as well as that of Fig. 3 are illustrations of systems in which continuous logs are not made. The arrangement of Figs. l and 2 makes a log like that illustrated at (d) in Fig. 4, while the arrangement of Fig. 3 makes a log like that shown at (e) in Fig. 4. In order to explain the purposes of the invention and to show why it is desirable to move the potential or pickup electrodes P1 and P2 with respect to the current electrode device C while a log is being made reference is had to the curves shown at (a), (b) and (c) of Fig. 4.

Curve (a) in Fig. 4 represents an ideal curve for any logging system which, insofar as is now known, is not capable of realization. It is what all inventors of electrical logging systems strive to achieve. Let it be assumed that there are a number of formations in the earth of different resistivity and that these formations have detinite boundaries. On the curve (a), the base portions designated 75, 76, 77, 78 and 79 represent strata such as shale of low resistivity while the kicks or detiections to the right, such as those designated 80, 81 and 82 represent formations of higher resistivity. The deiiection shown at 83 to the left of the lines 77 and 78 represents a formation of extremely low resistivity. Such a formation is sometimes said to produce a negative kick because it is to the left of the base. The dotted line shown at 84 represents a condition which often exists in a well bore and which an ideal system should retiect. The resistivity reading should be higher when the electrodes are spaced apart than it is when they are close together. A change in spacing of the electrodes should show the eifects of infiltration of mud fluid. In other words, the resistivity of the formation some distance away from the borehole as represented at 84, is usually greater than that near the borehole, as represented at 82, due to the invasion of the drilling mud into the formation near the borehole. Where the electrodes are spaced close together, they are not influenced greatly by the conditions existing some distance from the borehole so that the reading 82 is based primarily on infiltration. An ideal curve should show where there is marked iniiltration.

In the ideal curve (n) in Fig. 4 it is assumed that the magnitude of the deflections is proportional to the actual resistivity of the beds of the earth and hence indicative of the nature of the formations. It should be emphasized that this curve is only an ideal to be striven for. It docs not represent any particular resistivity log. The curve shown at (b) in Fig. 4 is illustrative of the type of curve obtained with a single electrode impedance log. As explained in the article by Guyod, cited above, a single point impedance curve is very good where used merely for the detection of boundaries between forma tions and this is clear from the illustration here. But the magnitude of a dellection does not accurately retiect the resistivity of a given formation, and for that reason the curve is not as valuable as some others in affording information for geological interpretation.

The curve shown at (c) in Fig. 4 is illustrative of the type of log obtained with the three-electrode system.

affectera While the three-electrode system is now standard, it is generally recognized by those familiar with the art that it is far from ideal in many respects. It is not good for the location of boundaries. Moreover, it is not accurate as to magnitude of deliection. In order to interpret such a curve correctly certain corrections must be made and these corrections depend upon the thickness of the particular beds (compare articles by Guyod and see the curves on pages 49 and 50 of the Oil Weekly for October 23, 1944).

The main cause of these defects or anomalies in the three-electrode system is the fact that when the current electrode passes a boundary between formations a disturbance is set up which affects the difference in potential across the pickup electrodes. In its most fundamental aspects, the present invention consists in providing means for removing the errors created when the current electrode passes a boundary. This is done by the simple expedient of holding, either actually or effectively, the current electrode stationary in the Well bore while the potential electrodes are moved to make the log.

Unfortunately, while the expedient of holding the current electrode still eliminates one of the fundamental diiiiculties in attempting to make an ideal log, it introduces other errors which have to be taken into consideration. In accordance with the present invention these other errors can also be corrected, but before describing what they are and how they are taken care of, the effects of holding the current electrode stationary will be set forth. The curves shown at (d) in Fig. 4 illustrate the type of log obtained when the arrangement of Fig. l and Fig. 2 is employed in a borehole. With this system the housing 16 which carries the current electrode device C is held stationary at a given level in the borehole while the potential electrodes P1 and P2 are pulled into the housing so that they aproach the current electrode device C.

There is, of course, a limit to the amount of cable that can be spooled in upon the reel 21. This reel must necessarily be small because of the limitations as to space in the borehole. For other reasons it is not feasible to have the potential electrodes more than twentyftve or thirty feet from the current electrode device C, one reason being that if the spacing is too great, the signal may become too small to be sent to the surface of the ground and be measured with the required accuracy.

Hence, in accordance with the arrangement of Figs. l and 2, only a few feet of borehole can be logged. It is then necessary to move the current electrode device to the new position. In other words, the logging proceeds step by step or a section at a time. However, this step by step process may be continued until as much of the borehole has been traversed as is desired. For example, let it be assumed that the cable 20 is thirty feet long. Let it also be assumed that when the conductors 20 are spooled up upon the reel 21 to about the amount indicated on the drawing in Fig. 1 the midpoint between the pickup electrodes Pi and P2 is ten feet from the current electrode device C. If the housing 16 is held in a stationary position while the potential electrodes P1 and P2 are spooled in as current is being supplied to the current electrode device C an electric log of twenty feet of borehole will be obtained in which there will have been eliminated any error due to variations in current distribution caused by the current electrode device passing a boundary. As soon as the twenty foot section of well bore has been logged, the conductors 20 may then be unspooled and the current electrode device moved to a point twenty feet above the initial position.

With the arrangement shown in Fig. 1 the more frequently the current electrode device C is moved from one depth to another the more accurate will be the log. For example, should the potential electrodes be moved only from thirty to twenty-eight feet, and then the current electrode device moved to a point two feet above the initial position and so on, proceeding step by step, a more accurate log will be obtained than when moved as indicated above. The reason for this is that as the potential electrodes approach the current electrode device an error is introduced due to the change in current density in the mud column depending upon the distance from the current electrode device. To illustrate this error the various logs shown on curve (d) of Fig. 4, which illustrate the type of curves produced by the arrangement of Figs. l and 2, are placed upon an inclined curve to the right. For example, starting at the bottom of curve (d) of Fig. 4, let us assume that the current electrode device C which is represented by a small rectangle is located as shown and that the potential electrodes P represented by two small circles are some distance below the current electrode device C, and that an electrical log is obtained as the potential electrodes are moved upwardly toward the current electrode device. Instead of drawing a straight line to conform with that indicated at 79 of the ideal curve (a) of Fig. 4, even though the resistivity remains constant the line of curve (d) inclines to the right as illustrated at 90. The incline is not even a straight line. It is a curve and follows the law that the current density decreases with the square of the distance the potential electrodes are from the current electrode device. lt is for this reason that if the step by step measurements are made frequently, ,so that the potential electrodes move only a small distance before the new curve is started the errors which creep into such curve (d) will be small.

As shown at 86 in Fig. 4 when the arrangement of Fig. l is employed to make a curve along a formation in which the mud from the borehole has invaded the formation to a limited extent the curve obtained will indicate the boundaries of the formation and to some extent indicate the amount of invasion. Here again, however, consideration must be given to the fact that if the resistivity had remained constant the log would nevertheless have described a curve.

The same errors appear at 87, 88 and 89 in curve (d) of Fig. 4. The boundaries are quite accurate and the magnitude of deflection is accurate if consideration is given to the fact that the log itself is superimpressed upon the curve.

It will be apparent that a log very closely approaching the ideal can be made if the inherent spurious deflections can be corrected or eliminated from curve (d) of Fig. 4. This can be done by changing the current supplied to the current electrode device C while the potential electrodes P1 and P2 are caused to approach the current electrode device C. Since the IR drop across any unit portion of the mud column in the borehole is inversely proportional to the square of the distance from the current electrode device the value of the current sent out from the electrode device C should be reduced according to the square of the travel of the potential electrodes P1 and P2. All that is required therefore is that a rheostat be inserted in series with the source of current and that this rheostat be so designed that its resistance increases with the square of the distance its pointer travels over it. Such a rheostat could obviously be incorporated in the arrangement of Figs. l and 2. For purposes of illustration, Fig. 3 of the drawing shows such a rheostat at 52 incorporated in the single conductor system there illustrated.

With thc arrangement shown in Fig. 3 logs like those shown on curve (e) of Fig. 4 are obtained. In other words, here is found a log in which the current electrode device is held stationary while the potential electrodes are moved, so as to eliminate any error due to changes in current density caused by the current electrode device passing boundaries of formations having different resistivity. At the same time there is correction for the errors due to change in distance between the electrodes.

Actually, of course, there are still some errors in the curve. The rheostat 52 must be designed to meet certain conditions. These conditions may not remain uniform in the well. For example, changes in resistivity of the mud itself at certain depths along the well bore introduce certain errors into the curves. Curve (e) of Fig. 4 shows that the magnitude of the deflection is very close to the apparent resistivities of the various formations, whether these be thick bed as shown at 81 in curve (a) of Fig. 4 or whether these be thin beds as shown at 80. A positive kick is as accurately indicated as a negative kick such as that shown at 83 of curve (a). Also it will be found possible with such an arrangement to pick out those formations in which mud invasion has occurred to a limited depth. Observe that the curve 91 of curve (e) has a different appearance at the top than it does at the bottom. This results from the fact that as the potential electrodes approach the current electrode device the amount of formation which is included within their effective zone is reduced. Hence as the potential electrodes approach the curent electrode device they are influenced more and more by the formation close to the borehole and less and less by the formation far away from the borehole. Of course, care will have to be observed in interpreting this log. Such a change might be caused by something other than invasion.

To be most effective, a continuous log rather than an intermittent or step by step log such as that shown at (e) of Fig. 4 should be provided. Figs. 5 to l2 of the drawing illustrate two different ways for bringing about the making of such continuous logs while retaining the advantages of curve (e) of Fig. 4.

In Fig. 5 the current electrode device and the potential electrodes are given the same reference characters as in Fig 1. Likewise these are shown associated with the housing 16 and the potential electrodes are shown mounted on a conductor cable 20 which is spooled upon a reel 21. In the arrangement illustrated a single conductor cable 92 is provided and this conductor cable is spooled upon the reel 93 at the surface of the ground, which is driven by suitable means 94. It will be understood that the conductor of the cable 92 is connected to a vacuum tube measuring device and a galvanometer as illustrated in detail in Fig. 8. Provision is made for causing the cable 92 to be spooled upon the reel 93 step by step and this movement of the cable 92 is brought ,about simultaneously and automatically with unspooling and spooling of the cable 20 upon the reel 21. The action is such that when the housing 16 and the current electrode device C are moved upwardly in the borehole the cable 20 is being unspooled, there being no electrical recording during this movement. Then with the housing 16 and thc current electrode device C remaining stationary in thc borehole electrical recordings are made while the potential electrodes P1 and Pz are moved upwardly by the reel 21.

To bring about the desired sequence of automatic step by step operation certain solenoid devices are mounted within the housing 16. Any suitable mechanism may be employed, but in the arrangement shown in Fig. 5 a solenoid operated ratchet 95 is employed to drive the reel 21 through suitable gearing designated generally 96. The solenoid which operates the ratchet wheel 95 is designated 97 and it is connected as will be described later in connection with the circuit of Fig. 8. Included in the mechanism for driving the reel 21 is a clutch 9S controlled by a sole noid 99 through a suitable yoke 100. The mechanism also includes a traveling limit switch 101 controlled by two stops designated 102 and 103. The limit switch 101 travels on a screw 104 xed to one of the gears 96. The upper part of the housing 16 of Fig. 5 includes a box 17 containing filters, transformers, rectitiers and so on, which form part of the electrical equipment lowered into the borehole. It should also be mentioned that the cable 20 must be fluid-sealed as it passes into the housing 16 to keep mud from preventing proper operation of the gear train and reel.

At the surface of the ground the mechanism for driving the reel 93 may include a hydraulic power ratchet drive mechanism 94 controlled by the relay coil 105 which controls a diaphragm valve (not shown). This mechanism must be such that it moves the housing 16 and the current electrode C approximately the same distance that the potential electrodes P1 and P2 move when the reel 21 draws them up toward the current electrode device C. In other words, for example, if the limit switch 101 causes the reel 21 to spool ten feet of cable 20 and then permits this amount of cable to unspool the drive mechanism 94 should be so designed as to raise the housing 16 ten feet each time that the current electrode device C is to be moved upwardly in the borehole. To this end a control disc such as that shown at 106 in Fig. 6 may be employed. This disc may be operated by a clock drive mechanism 107 or it may be rotated by hand or by any other suitable means. Time is not an essential factor in the mechanism, but a clock is a suitable control device. As shown in Fig. 6 the disc 106 is provided on its front surface with twenty pegs or pins which are designated 108. These operate a snap action micro switch 109. ln addition there is a single projection 110 on the periphery of the disc 106 which operates a second micro switch 111. With this arrangement it will be clear that the micro switch 109 is operated twenty times each revolution of the disc 106 while the micro switch 111 is operated once for each rotation of the disc 106. The micro switch 111 controls movement of the reel 93 at the surface of the ground while the micro switch 109 controls operation of the reel 21 located in the borehole as will be explained in connection with the circuit diagram of Fig. 8.

Fig. 7 illustrates an arrangement of a galvanometer recording system which may be used in connection with the apparatus of Figs. 5 and 8. The galvanometer itself which is designated 112 may be mounted upon a pivot 113 so that periodically it may be lifted about the pivot 113 by a solenoid 114. When so lifted the needle of the galvanometer 115 is lifted off of the stylus 116, there being a friction drive between the needle and the stylus as shown at 117. The friction is sufficient to prevent movement between the stylus and the needle when they are engaged. The stylus 116 is pivoted about an axle 118 immediately beneath the axle of the galvanometer needle 115. If the galvanometer is raised by the solenoid 114 so that the needle does not drive the stylus 116 the needle may then move to a new position before the galvanometer is again dropped so that the needle 115 may be reset upon the stylus each time that the galvanometer is lifted. By energizing the solenoid 114 each time that the current electrode device C is moved in the borehole the effect of the current electrode device passing a boundary between formations may be corrected for each time that the housing 16 is moved in the borehole. The chart 119 of Fig. 7 is driven by a ratchet wheel 120 operated by a solenoid 121. As will be described in connection with Fig. 8, the solenoids 114 and 121 are never operated at the same time, hence the paper or chart 119 does not move when the galvanometer is lifted up by the solenoid 114.

The circuit arrangement of Fig. 8 causes a certain sequence of operations of the system of Fig. 5. This sequence of operations is as follows:

l) After the apparatus has been lowered into the hole to a predetermined depth and with the cable 20 unwound from the reel 21 so that the potential electrodes are a known distance from the current electrode device C, the logging operation begins.

(2) As current is supplied to electrode device C, the potential electrodes P1 and P2 are spooled in on the reel 21 step by step. Simultaneously with each step movement of the potential electrodes, the chart at the surface of the ground is moved a step and the rheostat 52 adjusted a step to reduce the current sent out from electrode device C. Thus errors due to changes in distance between the potential electrodes and the current electrode device are compensated throughout the series of steps of this operation. Movement of the chart makes a log, the stylus being moved or held still by the galvanometer at each step, depending on the nature of the formations adjacent the potential electrodes.

(3) The reel 21 is now released and the electrodes P1 and P2 again fully extended. The cable 92 on the reel 93 is spooled at the same time so that the potential electrodes P1 and Pz do not actually move in the well bore, but the current electrode device does. No log is made at this time. The chart is not moved. The galvanometer is lifted so that its needle may move without moving the stylus from the position it occupied at the end of operation No. 2. Current of the intensity existing at the beginning of operation No. 2 is restored to current electrode device C by bringing the pointer 65 of rheostat 52 back to its starting position.

(4) The galvanometer needle is again set on the stylus, without moving the stylus from the position it occupied at the end of operation No. (2). Thus there is eliminated from the log any change caused by moving the current electrode device C.

Operations (2), (3) and (4) are then repeated in sequence until the desired portion of the bore hole is logged.

The circuit arrangement shown in Fig. 8 accomplishes these steps in the proper order. The operation may be almost entirely automatic. As in Fig. 3 the arrangement of Fig. 8 includes a 500 cycle source of alternating current designated 49 and a 60 cycle source of alternating current designated 50. The 500 cycle source is used to make the electrical log and the 60 cycle source is used to actuate the relays and solenoids to perform the desired sequence of connection and operations. A

The difference in potential picked up by the potential electrodes P1 and P2 is passed through a transformer 44, the connections being through slip rings 22 and 23 as well as brushes 24 and 25, the same as in Fig. 3. The secondary of the transformer 44 is connected to a rectifier 45 so that the signal passing upwardly through the single conductor 92 of the cable is a direct current voltage the potential of which is dependent upon the drop in alternating current of 50() cycle potential across the electrodes P1 and P2. By means of suitable filters the signal voltage, the 60 cycle voltage and the 500 cycle voltage are kept in the proper channels. The low pass filter is designated 48, the 60 cycle pass filter 57 and the 500 cycle pass filter 55, just as in Fig. 3. At the surface of the ground it is necessary to have one more filter than shown in Fig. 3. In Fig. 3 the 500 cycle current and the 60 cycle current are not passed down the line 46 at the same time, but in Fig. 8 both currents are impressed upon the line at the same time, and hence it is necessary to have a 60 cycle pass filter 122 in series with the 60 cycle source 50. The low pass filter is designated 58 and the 50() cycle pass filter designated 53 as in Fig. 3.

Also as shown in Fig. 3, there is a rheostat 52 associated with the chart driving mechanism. The arrangement is such that the pointer 65 of the rheostat makes one revolution thereon each time that the control disc 106 makes one complete revolution. Thus the pointer 65 is always at its starting position at the beginning of operation No. (2) mentioned above.

it may be necessary to change the position of the pointer 65 with respect to its shaft to maintain the proper current density in the vincinity of the potential electrodes. As explained herein, this adjustment is manual. Under some conditions of logging, this adjustment may not be necessary although the magnitude of the readings will not reach the extreme accuracy sought unless this adjustment is made.

In addition to the apparatus shown in Fig. 5, the circuit diagram of Fig. 8 also includes a relay coil 123 adapted 11 to cause its contact 124 to engage contact 125. The purpose of this relay will be apparent from the description of the system shown in Fig. 8.

ln consideringthe operation of the system shown in Fig. 8, let it be assumed that the apparatus within the housing 16 of Fig. 5 is lowered into the well and that the cable 20 is unwound from the reel 21. The 500 cycle current is being passed down through the cable 92 and out into the mud in the well bore through the current electrode device C. The D. C. signal from the pickup elcclrodes Pi and P2 is being passed upwardly through the cable 92, the low pass lter 58 and into the galvanometer vacuum tube meter 59 and thence to ground at 60. The rocking solenoid 114 is not energized so that the needle of the galvanometer is clutched to the stylus 116 and the stylus is resting upon the chart 119. Hence the apparatus is ready to start logging.

To perform operation No. (2) mentioned above, the control disc 106 is now rotated so that the micro snap switch 109 is caused to close each time that a pin 10S on the control disc strikes it. Each time that the switch 109 is closed current ows from the battery 126 to the relay coil 123 and then to ground at 127. Each time current also flows from the same source through the conductor 128 to the solenoid 121 and thence to ground at 129. Thus each time that the switch 109 is closed the chart advances one step and the pointer on the rheostat 52 advances one step.

When the relay 123 is energized the switch 124 strikes the contact 125 and thus connects the 60 cycle source S0 momentarily to the conductor 92 of the cable so that the 60 cycle source is impressed upon either solenoid 97 or 99 within the housing 16 in the well bore. The selection of which of the two solenoids 97 or 99 is energized dcpends upon the position of the limit switch 101. lf it is engaging the contact 102, which is the normal position and which is shown in Fig. 8, then the coil 97 is energized momentarily so as to actuate the ratchet wheel and rotate the reel 21 one step drawing the potential electrodes Pi and P2 upwardly toward the current electrode device C,

As the switch 109 is closed successively by the various contacts 108 on the control disc 106 this step by step movement of the electrodes P1 and P2 and the step by step movement of the chart 119 is continued until operation No. (2) is completed. To perform operation No. (3) the limit switch 101 shifts to engage contact 103 and the micro switch 111 is energized. At the same time that the micro switch 111 is energized the bar 108:1 on the control dise 106 holds the micro switch 109 closed for the same length of time that the micro switch 111 is closed. Thus while the bar 108a is holding the switch 1 09 closed 60 cycle current from the source 50 is continuously applied to the conductor 92 of the cable rather than merely causing pulses to be applied to the cable. Since the limit switch 101 is now engaging the contact 103 this 60 cycle source is applied to the solenoid 99 so that the clutch 98 is disengaged all during the time that the micro switch 111 is closed. Thus the cable 20 can pay otl from the reel 21 and the electrodes Pi and P2 can be extended. Actually they remain stationary while the housing 16 and the current electrode device C are moved upwardly in the well bore by the cable 92 being recled upon the reel 93.

As shown in Fig. 8 whenever the switch 111 is closed current may flow from the battery 126 through the coil and thence to ground at 130. The coil 105 is a relay coil which controls the operation of a diaphragm valve regulating suitable hydraulic power mechanism 94, the details of which are not shown in this specification being well known to those skilled in the art. The operation is such that whenever the coil 105 is energized the reel 93 is rotated to raise the housing 16 in the well bore. At the same time that the coil 105 is energized current from the battery 126 passes through the conductor 131 to the rocking solenoid 114 and thence to ground at 132. Thus all during the time that the cable 92 is being spooled upon the reel 93 the galvanometer 112 is lifted so that its needle 115 may move freely without moving the stylus 116. This is the operation while operation No. (3) in the sequence listed above is being performed. As soon as the reel 93 has raised the housing 16 the required distance the circuit at 111 is broken and the operation No. (2) of the step by step logging repeats.

The housing 16 should be raised approximately the same distance that the electrodes P1 and P2 moved while performing the logging operation No. (2). Errors in distance are not cumulative, so those due to variations in wrapping of the cable on the drum are not critical.

ln the arrangements thus far described it has been necessary actually to physically move the potential electrodes P1 and P2 with respect to the current electrode device C. The same result can be obtained by providing two current electrodes or rings in the current electrode device C and alternately switching from one current elec trode to the other to control the point of distribution of current therefrom between them. The two current electrodes are spaced vertically in the well bore. Thus step by step logging without actual physical relative movement taking place between the current electrode device and the pick-up electrode device. Due to the necessity for stuffing boxes and the like if the electrodes are actually moved relative to each other the making of step by step logs such as with the apparatus so far described is not without diiculties. Some of these are obviated by the system of Fig. l2.

To understand the sequence of steps in the operation of such a system, reference may be had to Fig. 9 where suitable current electrodes or rings and potential electrodes are diagrammatieally illustrated as associated with a galvanometer. For purposes of illustration let it be assumed that the two current rings C1 and C2 represented by small rectangles are six inces apart and that the two potential electrodes P1 and Pz represented by circles are also six inches apart and that these electrodes are all mounted on a xed structure so that the mid-points between the current rings and the potential electrodes are say lifteen feet apart.

Let it be assumed that the two current rings and the two potential electrodes are located in a well bore at say a depth of 5000 feet and that the needle of the galvanometer is pointing straight up as shown in step (a) Fig. 9. Current is being supplied to the lower current ring Cz in this step. If now the current is switched to the upper current ring C1, as shown at step (b) in Fig. 9 the galvanometer may deect. It may deflect in either direction depending upon the conditions along the borehole. As illustrated in step (b) of Fig. 9 the needle deliects to the right. This is indicated as a dashed line 135. It will be understood that in switching the current from the lower current ring C2 to the upper current ring C1, in passing from step (a) to step (b), none of the electrodes have moved physically in the well bore. Since the potential electrodes did not move the deflection shown at is false and is an error due to the shifting of the current from the lower to the upper current ring. Before the next step is performed, therefore, let it be assumed that the needle of the galvanonieter is subjected to angular adjustment with respect to its armature so as to bring the needle back to the full line position 136 in step (b). The needle is then at the correct reading for the position 0f the potential electrodes and the erroneous position of the armature is corrected.

Now, the next step can be performed. The potential electrodes and the current rings are moved upwardly in the well bore six inches. After this movement the current is switched back to the lower current ring C2. The lower current ring C2 is now in the same position in the well bore that the upper current ring C1 was during the avancee 13 performance of step (b). Any deflection of the galvanometer from that during step (b) is now a true deflection due to movement of the potential electrodes P1 and P2. (c) is therefore a correct reading of the change in resistivity between step (a) and step (c), and if it were recorded on a chart would describe a correct log. This operation may then be repeated.

In step (d) the electrodes are all held stationary in the well bore, that is at the same depth that they were during step (c), but the current is switched from the lower current ring C2 to the upper current ring C1. The false deection shown by the dashed line 138 (illustrated as to the left this time) is again observed and the galvanometer angularly adjusted to bring the needle back to the full line position 139 which is the same position it had before the switching occurred. This is the correct position to eliminate the error due to switching. Of course, the needle may go either direction at any time but so long as :it is brought back to the position it occupied before current was switched from the lower to the upper current ring the error is taken care of.

To perform step (e) in Fig. 9 all of the electrodes are then raised again another six inches and the current switched from the upper to the lower current ring. The lower current ring C2 is then again at the same depth in the well bore that the upper current ring was in performing step (d) so that current is supplied to the earth formations at the same point in the well bore in both steps (d) and (e). In step (e), it is indicated that the galvanometer has again returned to the same position it was during step (a). Hence the position of the needle during step (c) would indicate that in passing upwardly one foot in the well bore the potential electrodes passed a narrow or thin bed of highly resistant material in between two low resistivity beds. Thus errors due to movement of the current electrode device past either a more resistant or a less resistant bed are eliminated. It will be understood that the invention is not limited to any particular electrode spacing. However, the distances between the current rings should be the same as the distance between the potential electrodes and the readings should be made intermittently just after the electrodes are physically moved this sarne distance.

Thus in the arrangement of Fig. 9 the step by step logging is accomplished without physically changing the distance between the electrodes. The effective distance between the current electrode device and the pickup electrodes is changed by the switching action. In the appended claims, where changes in this distance are defined, it will be understood that both actual or physical changes and eiective changes are contemplated and that both are included in the definition and scope of the claims.

Figs. 10 and l1 illustrate apparatus which may be used in connection with the circuit diagram of the system shown in Fig. 12 for performing the desired sequence of operation illustrated in Fig. 9. In Fig. 10 the conductor of the cable upon which the electrodes are lowered into the well is shown at 92 as in Fig. 5 and the reel upon which the cable is spooled is designated 93. Any suitable means may be used to drive the reel. Its movement need not be intermittent as is the case in Fig. 5. In Fig. 10 a measuring Wheel 140 is shown engaging the cable 92. Mounted upon the measuring wheel 140 are suitable segments 141 which engage a microswitch 143. The measuring wheel 140 also is provided with buttons 144 adapted to engage switch blades 145 and 146. The functioning of these switches will be apparent from the description of Fig. 12.

The galvanometer arrangement of Fig. 11 is the same as that in Fig. 7 except that an additional electromagnet has been provided as shown at 147. The purpose of this electromagnet is to hold the stylus still when the' needle of the galvanometer is not engaging it. Such an electromagnet was not necessary in the arrangement of The position of the needle as shown at 137 in step Fig. 8 because in that case the chart did not move except when the galvanometer was making a log. In the arrangement of Fig. l2 the chart 119 moves steadily rather than with interrupted motion so that it is necessary to hold the stylus 116 stationary at times. The needle 115 of the galvanometer grasps the stylus 116 only intermittently to move it from one position to another.

In the arrangement shown in Fig. 12 the parts which are similar to those of Fig. 8 are similarly designated and it will not be necessary to repeat here their manner of operation. Instead of there being only one ring used as the current electrode device as shown in Fig. 8 there are now two current electrodes or rings designated C1 and C2. These rings constitute a single current electrode device. They are spaced apart the same distances as the potential electrodes P1 an P2 and are alternately connected into the circuit of the 500 cycle source by means of a switch 150 controlled by a relay 151 operated by 6() cycle current. Normally the switch 150 is engaging its lower contact 152 as shown but whenever the relay coil 151 is energized the switch blade 150 breaks the circuit to the lower current ring C2 and engages its upper contact 153 to make the circuit to the upper current ring C1. This happens whenever 60 cycle current is passed down through the conductor 92 of the cable. At the surface of the ground the 60 cycle source is connected to its 60 cycle pass iilter 122 through a switch 154 having a contact 155. The closing of this switch is controlled by a relay coil 156 connected to the contact 146 associated with the measuring wheel shown in Fig. 10. Since the buttons 144 on the measuring wheel 140 close the circuits to the contacts and 146 only temporarily it will be obvious that the circuit to the relay coil 156 closes only temporarily. It will also be observed that the measuring wheel 140 rotates in a counterclockwise direction as the cable 92 is being spooled upon the reel 93. Hence the button 144 engages contact 145 just prior to the time it engages the contact 146.

When the contact 145 engages button 144 current flows from the battery 157 to energize the solenoid 121. This drives the chart through the ratchet wheel 120.

It should be observed that Fig. 12 employs no rheostat like that shown at 52 in Fig. 3 and Fig. 8. Instead the correcting device consists of a small resistance 158 which is placed in series with the lower current ring C2 in the borehole. If the potential electrodes P1 and P2 are located some distance away from the current electrode device this resistor 158 will not be necessary. However, it serves in some extent to compensate for the changes in current density caused by the switching operation between the current rings.

The circuit for the electromagnet 147 of Fig. l1 is shown in Fig. 12. This electromagnet is merely connected in series with a battery 159 and the switch 143 operated by the measuring wheel 140. Whenever the segments 141 engage the microswitch 143 and hold it closed the stylus of the recording galvanometer is held stationary. As will be seen from an inspection of Fig. l0 this holding action occurs over most of the measuring wheel 40. Of course, there may be as many segments 141 on the measuring wheel as desired and as many buttons 144 as desired but, as shown, the stylus should be held stationary except when the switch 145 is closed.

In the circuit diagram of Fig. 12 the parts are shown in the position which they occupy while an actual recording is being made upon the chart 119. That is, the switch 144 is engaging the contact 145; the switch 154-155 is open, the switch 143 is open so that the galvanometer is capable of moving the stylus 116 to make a record upon the chart 119 while the solenoid 121 actuates the chart 119 through the ratchet wheel 120.

The source of 500 cycle current designated 49 is connected to the conductor 92 at all times so that, with the switches in the position shown, current is being sent out from the lower current ring C2. The signal from the two potential electrodes Pi and P2 is being sent up the conductor 92 as a direct current which is impressed upon the vacuum tube voltmeter 59 to cause the galvanometer to move the stylus in accordance with the resistivity of the formations. As soon as the measuring wheel 140 rotates a small amount from the position shown in Fig. the contact button 144 will engage the switch blade 146 just as the button leaves the blade 145. At the same instant the microswitch 143 will close so that the stylus will be held stationary until the next button 144 engages the blade 145 and the parts are back in the position shown in Figs. 10 and 12.

When the buttons 144 engage the blade 146 the relay coil 156 will be energized so that 60 cycle current will pass down the borehole and cause switch 150 to engage contact 153. The 500 cycle current will then be impressed upon the formations in the borehole from the upper current ring C1. The signal potential across the potential electrodes P1 and Pz will be transmitted to the vacuum tube voltmeter 59 and to the galvanometer therein but inasmuch as at that instant the rocking solenoid 114 is also energized by the battery 157, the needle 115 of the galvanometer cannot move the stylus 116. The needle will take a new position above the stylus 116 depending upon the amount of detiection created by shifting the current from the lower ring C2 to the upper current ring C1. The Width of the blade 146 is sufiicient to give the galvanometer sufficient time to reach a stable position above the stylus 116 during this step in the operation. As soon as the button 144 leaves the blade 146, the rocking solenoid 114 is deenergized so that the needle again frictionally engages the stylus 116. The galvanometer is then set for making another record when the button 144 again engages the contact 145. Of course, as soon as the circuit between the button 144 and the blade 146 is broken the relay 156 is also deenergized so that the blade 150 in the borehole again engages the contact 152 and the connection of the 500 cycle source is again on the lower current ring Cz.

There must be a definite correlation between the spacing of the buttons 144 and the segments 141 on the measuring wheel 140 and the distance between the potential electrodes in the borehole. Since the galvanometer needle 115 engages the stylus 116 while the electrodes in the borehole are in motion it is necessary to have electromagnet 147 hold the stylus stationary until the electrodes have moved the exact distance necessary to bring the lower current electrode into the same position in the borehole that the upper current electrode occupied when the galvanometer was lifted by the rocking solenoid 114 at the start of the movement of the electrodes.

in considering the operation of the system of Fig. 12 1ct it be assumed that the electrodes are in the position shown at step (a) in Fig. 9; that the connections are as shown in Fig. l2 and that the galvanometer deflection is as shown at (a) in Fig. 9. As the measuring wheel 140 rotates in a counter-clockwise direction the button 144 engages thc blade 146 so that the connections shown in step (b) of Fig. 9 are effected. Of course, there is vertical movement of the electrodes between steps (a) and (b) but this may be small and may be ignored. As contact button 144 engages blade 146 the stylus of the recorder is held stationary so that the indication 136 of the needle in step (b) of Fig. 9 is maintained insofar as any recording is concerned. The galvanometer needle may deflect as shown by the dashed line 135 but the stylus itself remains stationary until the electrodes move upwardly in the borehole to the position shown in step (c) in Fig. 9. Long before the electrodes reach this position the rocking solenoid 114 releases the galvanometer (since the solenoid 114 is energized only when button 144 engages blade 146) so that galvanorneter needle 115 is again engaging the stylus 116. When the electromagnet 114 is not energized, the needle 115 is frictionally engaged with stylus 116 and does not move with respect to it. Hence the needle and the stylus cannot deect until the current is switched to the lower current ring Cz again, for the stylus is held by the electromagnet 147 until that time due to the switch 143 be ing closed. Thus while the electrodes are moved through the borehole continuously the readings are taken intermittently, each reading being taken only after the galvanometer needle has swung free from the stylus for an instant to take care of changes in potential caused by switching from one current ring to the other when the current rings are on opposite sides of a formation boundary. While the graph upon the chart 119 is a continuous line, actually it is based upon a number of readings made successively. Each reading draws a small ver tical line upon the chart but inasmuch as the stylus never leaves the chart a continuous graph is produced. There is, of course, a slight error in the graph for this reason, like any curve plotted on the basis of a number of determined points upon it. The best plotted curve is one which has a large number of determined points. However, there are factors limiting the number of determined points or readings in the present case. Time is required for the galvanonieter to reach a stable position at each reading and time is required for the movement of the relay switches and solenoids. Provision also has to be made to prevent transient voltages from unduly affecting the readings. All of lthese considerations impose limitations upon the speed of logging which is a disadvantage. Logs must be made rapidly to avoid the danger of blow-outs. Hence, as is often the case, there are conflicting factors involved in the question of the best distance between the current rings and the frequency of the readings, with this system.

In all of the embodiments disclosed herein the arrangement has been applied to what is known as a three electrode logging system. As indicated above, invention is not limited to any number of electrodes. For example, a two electrode system can be provided by merely grounding one of the pick-up electrodes P1 or P2 on the sheath of the cable. Where both pick-up electrodes are used and the potential between them read at the surface, one may be said to constitute a reference point for the other. Where only one is employed, the sheath may constitute the reference point. Any of the known multiple electrode configurations may be employed both as to number of electrodes and the distance of separation between them. Moreover, the system may also be used where the potential electrode or electrodes are maintained stationary while one or more current electrodes are moved during the making of the log. The method requires only that either the current or the potential electrodes remain in a fixed position while the measurements are being made. Any number of successive connections might be made to various electrode configurations within the borehole either while the housing for the electrodes is maintained stationary or while it is being moved.

Thus while only a few embodiments of the invention have been shown and described herein it is obvious that various changes may be made without departing from the spirit of the invention or the scope of the annexed claims.

l claim:

l. In a system for making a resistivity log in a borehole in the earth, the combination of an electrical cable, an electrode assembly supported in the borehole on the cable and including a current electrode device and a pickup electrode, a source of current at the surface of the ground connected to said cable for causing current to llow from the current electrode device into fluid in the borehole and into the earth formations, measuring equipment at the surface of the ground, also connected to said cable for taking readings of the electrical potential between the pick-up electrode and a reference point, an arrangement for changing the effective distance between the current electrode device and the pick-up electrode 17 while the readings are being taken to enable the making of a step by step log and a control circuit connected to said cable and said measuring equipment for correlating the changes in said effective distance with said readings.

2. In a system for making a resistivity log in a borehole in the earth, the combination of an electrical cable, an electrode assembly supported in the borehole on the cable and including a current electrode device and a pick-up electrode, a source of current at the surface of the ground connected to said cable for causing current to ow from the current electrode device into fluid in the borehole and into the earth formations, measuring equipment at the surface of the ground, also connected to said cable for taking readings of the electrical potential between the pick-up electrode and a reference point, an arrangement for changing the effective distance between the current electrode device and the pick-up electrode while the readings are being taken to enable the making of a step by step log, a control circuit connected to said cable and said measuring equipment for correlating the changes in said distance with said readings and a correcting device including a resistance element for varying the amount of current flowing from the current electrode device depending upon changes in effective distance bctween it and the pick-up electrode,

3. In a system for making a resistivity log in a borehole in the earth, the combination of an electrical cable, an electrode assembly supported in the borehole on the cable and including a current electrode device and a pick-up electrode device, a source of current at the surface of the ground connected to said cable for causing current to ow from the current electrode device into fluid in the borehole and into the earth formations, measuring equipment at the surface of the ground also connected to said cable for taking readings of the electrical potential between an electrode of the pick-up electrode device and a reference point, a reel incorporated in said electrode assembly, a conductor spooled upon the reel and connected to one of said electrode devices whereby the distance between the electrode devices can be changed while the readings are being taken to enable the making of a step by step log and a circuit for controlling movement of said reel connected to said cable and said measuring equipment for correlating the changes in distance between said electrode devices with the readings.

4. in a system for making a resistivity log in a borehole in the earth, the combination of an electrical cable, an electrode assembly supported in the borehole on the cable and including a current electrode device and a pick-up electrode device, a source of current at the surface of the ground connected to said cable for causing current to flow from the current electrode device into fluid in the borehole and into the earth formations, measuring equipment at the surface of the ground also connected to said cable for taking readings of the electrical potential between an electrode of the pick-up electrode device and a reference point, a reel incorporated in said electrode assembly, a conductor spooled upon the reel and connected to one of said electrode devices whereby the distance between the electrode devices can be changed while the readings are being taken to enable the making of a step by step log, a circuit for controlling movement of said reel connected to said cable and said measuring equipment for correlating the changes in distance between said electrode devices with the readings and a correcting device including a resistance element for varying the amount of current owing from the current electrode device depending upon the changes in distance between it and the pick-up electrode device.

5. ln a system for making a resistivity log in a borehole in the earth, the combination of an electrical cable, an electrode assembly supported in the borehole on the cable and including a current electrode device having two electrodes spaced vertically and electrically connected to the cable and a pick-up electrode electrically connected to the cable, a source of current at the surface of the ground connected to said cable for causing current to flow from the current electrode device into huid in the borehole and into the earth formations, measuring equipment at the surface of the ground connected to said pick-up electrode through said cable for taking readings of the electrical potential between the pick-up electrode and a reference point, an electrical switch associated with said current electrode device for alternating the point of distribution of current therefrom between the two electrodes thereof, thereby making it possible effectively to change the distance between the current electrode device and the pick-up electrode while the readings are being taken to enable the making of a step by step log and a control circuit having a switch actuator for said switch connected to said cable and said measuring circuit making possible correlation of effective changes in distance between said current electrode device and said pick-up electrode with the readings.

6. In a system for making a resistivity log in a borehole in the earth, the combination of an electrical cable, an electrode assembly supported in the borehole on the cable and including a current electrode device having two electrodes spaced vertically and a pick-up electrode device having two pick-up electrodes, a source of current at the surface of the ground connected to said cable for causing current to ow from the current electrode device into fluid in the borehole and into the earth formations, measuring equipment at the surface of the ground also connected to said cable for taking readings of the electrical potential across pick-up electrodes of said pick-up electrode device, a reel incorporated in said electrode assembly, conductors spooled upon said reel and connected to said pick-up electrode device whereby the distance between said pick-up electrode device and the current electrode device can be changed while the readings are being taken to enable the making of a step by step log and a circuit for controlling movement of said reel connected to said cable and said measuring equipment for correlating the changes in distance between the pick-up electrode device and the current electrode device with the readings.

7. In a system for making a resistivity log in a bore hole in the earth, the combination of an electrical cable, an electrode assembly supported in the borehole on the cable and including a current electrode device and two pick-up electrodes, a source of current at the surface of the ground connected to said cable for causing current to ow from the current electrode device into fluid in the borehole and into the earth formations, measuring equipment at the surface of the ground also connected to said cable for taking readings of the electrical potential across said pick-up electrodes, a reel incorporated in said electrode assembly, conductors spooled upon said reel and connected to said pick-up electrodes whereby the dis tance between said pick-up electrodes and the current electrode device can be changed while the readings are being taken to enable the making of a step by step log, a circuit for controlling movement of said reel connected to said cable and said measuring equipment for correlating the changes in distance between the pick-up electrodes and the current electrode device with the readings and a correlating device including a resistance element for varying the amount of current owing from the current electrode device depending upon the changes in distance between it and the pick-up electrodes.

8. In a system for making a resistivity log in a bore hole in the earth, the combination of an electrical cable, an electrode assembly supported in the borehole on the cable a current electrode device having two vertically spaced current electrodes connected to the cable and two pick-up electrodes connected to the cable, a source of current at the surface of the ground connected to said cable for causing current to flow from the current electrode device into fluid in the borehole and into the earth formations, measuring equipment at the surface of the ground connected to said pick-up electrodes through said cable for taking readings of the electrical potential across said pick-up electrodes, an electrical switch associated with said current electrode device for alternating the point of distribution of current therefrom between the two current electrodes thereof, thereby making it possible effectively to change the distance'between the current electrode device and the pick-up electrodes while the readings are being taken to enable the making of a step by step log and a control circuit having a switch actuator for said switch connected to said cable and said meas uring circuit for making possible correlation of effective changes in distance between the current electrode device and the pick-up electrodes with the readings.

9. The method of making a resistivity log in a borchole in the earth which consists in lowering into said borehole an elongated insulated body supporting a potential electrode adapted to be moved axially with respect to said elongated insulated body, said elongated insulated body also having a current electrode mounted thereon causing an electrical current to flow from said current electrode through the earth formations, picking up, at a point in the fluid spaced from the point `of current distribution, a potential caused by the flow of current, mak ing measurements of said potential, changing the effective distance between the points during the making of the measurements, correlating the changes in this effective distance with the changes in measurements to determine the effect of the changes in effective distance and varying the amount of current flowing from said current electrode depending upon said changes in effective distance to correct for changes in current density caused by said changes in effective distance.

10. In a system for making lateral measurements within a well borehole, apparatus comprising: an elonI gated insulated supporting body adapted to be lowered axially into a well borehole on a conductor cable; an inl put electrode carried on said body; means supported by said body to pick up potential dierences occurring be tween a pair of substantially fixedly axially spaced-apart localities in the well borehole which are separated axially from said input electrode; means mounted in said elongated insulated supporting body to vary the effective axial separation between said input electrode and said pair of axially spaced localities from which said potential differences are picked up; and electrical connections for connecting said pickup means to insulated conductors in said conductor cable.

ll. In a system for making lateral measurements within a well borehole, apparatus comprising: an elongated insulated supporting body adapted to be lowered axially into a well borehole on a conductor cable; an input electrode carried on said body; means supported by said body to pick up potential differences occurring between a pair of substantially iixedly, axially spacedapart localities in the well borehole which are separated axially from said input electrode; means mounted in said elongated insulated supporting body to vary the effective axial separation between said input electrode and said pair of localities from which said potential differences are picked up; means to modify thus picked-up potential differences substantially to eliminate differences thereof at different pickup localities which would be solely the function of the said variation of the said effective axial separation under conditions where an electric field extends substantially radially in all directions from said input electrode as a center, through surrounding material of uniform resistivity and electrical connections for introducing such modified potentials into insulated conductors in such conductor cable,

12. In a system for making lateral measurements within a well borehole, apparatus comprising: an elongated insulated supporting body adapted to be lowered axially into a well borehole on a conductor cable; an input electrode carried on said body; means supported by said body to pick up potential differences occurring between a pair of substantially fixedly, axially spaced-apart localities in the Well borehole which are separated axially from said input electrode; means mounted in said elongated insulated supporting body to vary the effective axial separation between said input electrode and said pair of axially spaced-apart localities from which said potential differences are picked up; means to modify thus picked-up potential differences substantially to eliminate changes thereof which would be solely the function of the said variation of the said effective axial separation under conditions Where an electric field extends substantially radially in all directions from said input electrode as a center, through surrounding material of uniform resistivity; electrical con nections for introducing such modified potentials into insulated conductors in such conductor cable; and an electrical measuring means connected to the said conductors for indicating values representative of the said modified potential differences with respect to values indicative of depth of the localities within a well borehole at which such potential differences occur.

13. Apparatus for electrical well logging which comprises, in combination, a generating electrode, a pickup electrode, means including an elongated body carrying said electrodes in axially spaced-apart relation for suspending said electrodes in a well bore, and means carried by said body member for varying the spacing between said electrodes when they are suspended in the well bore.

14. In a system for making electrical measurements Within a well borehole, apparatus comprising: an elongated body adapted to be lowered into a. duid-containing well borehole by means of a conductor cable; an input electrode on said body and adapted to make electrical contact with conductive uid in such borehole; a pair of pickup electrodes carried on said body, the pickup electrodes of said pair being spaced a fixed distance apart substantially axially with respect to one another, and said pair of pickup electrodes being movable substantially axially relative to said input electrode; means to impart such relative motion to said electrodes; and electrical connections from each of said electrodes to separate insulated conductors in said conductor cable.

15. A method of making electrical measurements within a borehole comprising: positioning an elongated insulated body having an input electrode within the well borehole; establishing a ground connection remote from said input electrode; liowing an electric current through the earth formations surrounding said borehole between said input electrode and said ground connection to establish thereby a potential gradient; picking up with potential electrodes movably supported by said elongated insulated body the resultant potential differences between two substantially axially spaced-apart localities in the borehole in the vicinity of said input electrode while changing the axial distance of said localities from said input electrode in said borehole and while maintaining the spacing between said spaced-part localities substantially constant; modifying said picked up potential differences substantially to eliminate changes thereof which are solely the function of the said changes in distance of said spaced localities from said input electrode; and measuring the thus modified potential differences in correlation with said changing distance from said input electrode.

16. A method of making electrical measurements Within a well borehole comprising: positioning an elongated insulated body having an input electrode and a pair of potential pickup electrodes supported by said body and adapted to be moved axially with respect t0 said body within the well borehole; establishing a ground connection remote from said input electrode; flowing an electric current through the earth formation surrounding said borehole between said input electrode and said ground connection to establish thereby a potential gradient; locating said pair of potential pickup electrodes in said well borehole in the vicinity of said input electrode, said pickup electrodes of said pair being spaced apart axially with respect to one another and with respect to said input electrode; moving said input electrode and said pair of pickup electrodes together longitudinally through a length of said borehole; simultaneously moving said pair of pickup electrodes axially with respect to said input electrode to vary the distance of said pair of pickup electrodes axially from said current input electrode while maintaining the axial spacing between said pickup electrodes of said pair constant; modifying the resultant potentials picked up by said pickup electrodes substantially to eliminate differences therein which are solely the function of the said variation in the distance of said pickup electrodes from said input electrode; utilizing the thus modified potentials picked up between the said potential pickup electrodes of said pair for continuously recording values representative of the potential diierences picked up between said pair of pickup electrodes with respect to values representative of corresponding distances between said pickup electrodes and said input electrode while said pair of pickup electrodes are moving axially with respect to said input electrode as beforementioned; and correlating said continuous recording with the depth of said moving electrodes within said well borehole.

References Cited in the le of this patent UNITED STATES PATENTS 2,117,390 Zuschlag May 17, 1938 2,133,786 Neuield Oct. 18, 1938 2,192,404 Jakosky Mar. 5, 1940 2,206,890 Hawley July 9, 1940 2,293,024 Klipsch Aug. 11, 1942 2,295,738 Gmbergh sept. 15, 1942 2,317,259 Doll Apr. 20, 1943 2,393,009 Chun Jan. 15, 1946 2,397,255 Ennis Mar. 26, 1946 2,404,622 Doan July 23, 1946 2,414,899 Rust Jan. 28, 1947 2,415,364 Mounce Feb. 4, 1947 2,415,636 Johnson Feb. 11, 1947 FOREIGN PATENTS 370,979 Italy May 6, 1939 

1. IN A SYSTEM FOR MAKING A RESISTIVITY LOG IN A BOREHOLE IN THE EARTH, THE COMBINATION OF AN ELECTRICAL CABLE, AN ELECTRODE ASSEMBLY SUPPORTED IN THE BOREHOLE ON THE CABLE AND INCLUDING A CURRENT ELECTRODE DEVICE AND A PICKUP ELECTRODE, A SOURCE OF CURRENT AT THE SURFACE OF THE GROUND CONNECTED TO SAID CABLE FOR CAUSING CURRENT TO FLOW FROM THE CURRENT ELECTRODE DEVICE INTO FLUID IN THE BOREHOLE AND INTO THE EARTH FORMATIONS, MEASURING EQUIPMENT AT THE SURFACE OF THE GROUND, ALSO CONNECTED TO SAID CABLE FOR TAKING READINGS OF THE ELECTRICAL POTENTIAL BETWEEN THE PICK-UP ELECTRODE AND A REFERENCE POINT, AN ARRANGEMENT FOR CHANGING THE EFFECTIVE DISTANCE BETWEEN THE CURRENT ELECTRODE DEVICE AND THE PICK-UP ELECTRODE WHILE THE READINGS ARE BEING TAKEN TO ENABLE THE MAKING OF A STEP BY STEP LOG AND A CONTROL CIRCUIT CONNECTED TO SAID CABLE AND SAID MEASURING EQUIPMENT FOR CORRELATING THE CHANGES IN SAID EFFECTIVE DISTANCE WITH SAID READINGS. 